Anaerobic digestion refers to both a natural microbial process, which takes place in the absence of oxygen, and, an engineered process, which utilizes the microbial process. Both produce methane gas (CH4) as an end product. Anaerobic digestion is of great interest today due to its potential as a renewable energy source.
There is much confusion regarding the use of the terms “stage” and “phase” in the anaerobic digestion literature. Numerous authors have used the terms interchangeably. However, as used herein, the term “phase” is used to refer to biological steps in the anaerobic digestion process, whereas the term “stage” refers to an engineered physical entity (e.g., tank, container) used to contain the microbial phases.
The term “feedstock,” also referred to as influent, refers to liquid and solid material fed into a an anaerobic digester, including but not limited to dairy manure/waste, municipal and industrial waste water sludge, organic material, biomass waste, biodiesel production waste, ethanol production waste, and food processing waste.
Anaerobic digestion is a complex process, mediated by a diverse array of microorganisms in the absence of oxygen. During anaerobic digestion, these microorganisms digest organic matter and produce methane gas as an end product. The complexity of the anaerobic microbial community is illustrated by data identifying over 9,000 active species in wastewater sludge digesters (Curtis (2002)).
Anaerobic digestion has been described as a three phase process (Geradi (2003)), a four phase process (Schink (1992); Deublein and Steinhauser (2008); Khanal (2008)), a five phase process (Liu and Ghosh (1997)), and a nine phase process (Pohland (1992)). These great variations in how the literature defines the number of phases present clearly indicates the complexity of the microbial systems involved.
Most recently, a four-phase process, constituting a food chain, has been generally accepted as a working model (Deublein and Steinhauser (2008); Khanal (2008)). These four phases consisting of a Hydrolysis Phase, an Acidogenesis Phase, an Acetogenic Phase, and a Methanogenesis Phase.
The Hydrolysis Phase is the first phase. The Hydrolysis Phase involves the digestion of complex carbohydrates, proteins and lipids into simpler substrates such as sugars, amino acids and fatty acids. It is analogous, in many ways, to the functions carried out by the stomach in mammalian digestive systems. Hydrolysis bacteria include both facultative anaerobic microorganisms (able to live under aerobic as well as anaerobic conditions) and strictly anaerobic microorganisms. Hydrolysis bacteria tend to be highly resistant to environmental fluctuations such as temperature and pH changes, thrive in an acidic environment, have high reproductive rates and growth rates, and are not usually adversely affected by toxins and heavy metals which may be present in the feedstock. Since the hydrolysis step is required to treat raw particulate matter, it often is a rate-limiting step in the anaerobic process due to the difficulty of digesting these often complex substrates (Sanders, et al. (2000); Zeeman and Sanders (2001); Sanders (2002); Gomec, et al. (2003); Gosh (1985)). Improved mixing and particulate disruption approaches can go far to minimizing this potential limiting problem (Sanders, et al. (2000); Palmowski, et al. (2003)), as has been shown in a recent report on the effect of optimizing sludge digester mixing (Marx, et al. (2007)).
The Acidogenesis Phase is the second phase in the anaerobic food chain. The Acidogenesis Phase involves another group of both facultative and strictly anaerobic bacteria that, utilizing the simple substrates provided by the hydrolysis bacteria, metabolize these secondary compounds into water soluble organic acids, alcohols, and carbon dioxide and hydrogen gas (Britz, et al. (1994); Yu, et al. (2003)). One study identified two hundred and eighty eight (288) different strains of acidogenic microbes in four anaerobic digesters in South Africa (Britz, et al. (1994)), illustrating the complexity of this phase.
The Acetogenic Phase is the third phase in the anaerobic food chain. In the Acetogenic Phase, homoacetogen bacteria utilize the products produced by the prior Acidogenesis Phase acidogens. The homoacetogen bacteria produce water-soluble acetate, an important precursor to methane formation (Deublein and Steinhauser (2008); Khanal (2008)).
The Methanogenesis Phase is the fourth and final phase. The Methanogenesis Phase results in the production of methane gas (CH4). Methane producers are not true bacteria, but belong to an ancient group of microorganisms termed the Archaea. Recent evidence indicates that methanogens were active 3.5 billion years ago (Uneno, et al. (2006)). There are numerous species of methanogens capable of metabolizing a variety of low molecular weight water-soluble organics and gases. Methanogens are among the most strictly anaerobic organisms known, their growth being inhibited by the presence of even extremely small amounts of oxygen. Methanogens also are slow in reproducing, prefer a basic pH, and tend to be negatively affected by potential toxins such as heavy metals, solvents, pesticides and herbicides. Methanogens are also adversely affected by relatively small changes in environmental factors, such as pH and temperature. Most of the reputation of anaerobic digesters for instability, measured by the cessation of biogas production, can be traced to a failure of the methanogen populations.
The natural biological processes described above have been used extensively in an engineered application for over 100 years, long before the intricate biological relationships were understood. Said application has been almost exclusively at wastewater treatment plants for the stabilization and volume reduction of sludges. The production of energy has not been the primary goal of these systems. There are approximately 16,000 individual anaerobic digestion tanks operating in the United States alone. These tanks range in size from several hundred thousand gallons to several million gallons.
The vast majority are single stage systems where the four biological phases are forced to operate in a single tank. This creates numerous operational problems.
First, the hydrolysis bacteria and acidogenic bacteria (acidogens) have pH optimums of 5.5 to 6.5; whereas the methanogenic bacteria (methanogens) have pH optimums of 7.8 to 8.2 (Khanal, 2008). This presents challenges with using a single stage reactor (digestion tank) because hydrolysis begins immediately when the raw organic feedstock enters the digestion tank. Hydrolysis causes a rapid drop in pH as acidic products such as organic acids are rapidly produced. This acidic pH in turn inhibits the growth and metabolic activity of the methanogens.
To counteract this, a buffering agent (e.g., lime) must be added to the digestion tank to raise the pH to 7.8 to 8.5, the optimum pH for methane (CH4) production. This pH adjustment must be estimated and performed manually because the quantity of buffering agent required will depend upon multiple factors, including, but not limited to, the feed rate and the chemical characteristics of the undigested organics in the feedstock. Due to the size of these reactors, substantial quantities of buffering agent are needed to adjust the pH. Since the hydrolysis phase is facilitated by acid conditions, raising the pH to satisfy the requirement of the methanogens can inhibit the rate of hydrolysis, making operation of the digester a precarious balancing act requiring trained and alert operators. No matter how skilled the operator is, effectively combining efficient digestion and energy production has been virtually impossible in such a conventional digester.
Second, methanogenic organisms are slow reproducers and do not compete well for attachment space with the more robust and aggressive hydrolysis and acidogenic populations.
Third, in order to achieve the higher temperatures favored by methanogens, the contents of the entire digestion tank must be heated via a heating means (e.g., heater) to 30° C. to 38° C. for mesophilic operation or 49° C. to 57° C. for thermophilic operation, at which latter temperature range the highest rates of methane (CH4) production are achieved. Due to the large tank sizes typically used, these elevated temperatures require the utilization of significant amounts of energy (to heat the digestion tank), often reducing the net energy output of the anaerobic digestion system by as much as fifty percent (50%) or more.
Fourth, heavy metals or other toxins introduced into the single reactor with the feedstock come into immediate and direct contact with the environmentally sensitive methanogens. This is a frequent contributor to digester problems and reduction or cessation of methane (CH4) production.
Fifth, each time digested solids are discharged from the single digestion tank, a portion of the valuable, but slowly reproducing, methanogens, which are attached to the solid particles, are also lost.
Sixth, methane gas (CH4) produced by conventional anaerobic digesters has a high carbon dioxide (CO2) content, often totaling 30 to 40 percent or more. For this reason, it has a lower BTU value than natural gas, and is referred to as “biogas.” Carbon dioxide is a food source for methanogens, and thus the presence of CO2 in the biogas is an indication of conversion inefficiency in single stage and two stage anaerobic digesters.
Seventh, these operational challenges require a highly trained and attentive operational staff to properly operate conventional digesters. Such staff is in short supply.
The above items are the main reason why anaerobic digestion has not progressed more widely as a reliable source of renewable energy.
In an attempt to solve these problems, various multiple stage reactor configurations, using two or more separate tanks, have been proposed. Two stage reactor designs attempt to isolate the hydrolysis/acidogenesis phase in the first tank, and the methanogenic phase in a second tank. This is based on the well-established fact that the food for the methanogens is water-soluble.
In addition, three-stage and even four-stage reactor configurations have been proposed. However, none of these have solved the operational sensitivity problems, nor have they significantly increased biogas yields or biogas purity as evidenced by the low numbers of full-scale multi-stage installations which have been constructed. Single stage digesters are still the norm.
As a potential source of renewable energy, anaerobic digestion has a number of distinct advantages over other biofuels, such as ethanol or biodiesel.
First, it produces energy from existing waste organics (e.g., animal manure, municipal solid waste, food processing waste, wastewater treatment sludge, process sludge from such industries as ethanol production, biodiesel production, and paper mills). There are enormous quantities of these waste organics readily available.
Second, in deriving energy from these waste organics, anaerobic digestion also performs a significant role in ground water protection, odor control, and greenhouse gas reduction.
Third, anaerobic digestion can be used to produce energy from biomass crops.
Fourth, anaerobic digestion does not require energy intensive drying prior to digestion.
Fifth, there is a large, albeit inefficient, pre-existing installed base of single stage digesters, for instance it has been estimated that there are 12,000 to 16,000 individual digester tanks in the United States and over 20,000 in Europe. This installed base provides engineering and operational expertise on construction, operation, safety and utilization issues for the produced methane gas (CH4). Additionally, the installed base is ripe for retrofitting with technological enhancements aimed at increasing methane gas (CH4) production.